@ PSI Experiment The Mu3e

Tau 2018 Amsterdam Sept. 27, 2018

The Mu3e Experiment

@ PSI

searching for the neutrinoless muon decay m

⁺

→ e

⁺

e

^-

e

⁺

Alessandro Bravar

for the Mu3e collaboration

LFV in the “Standard Model”

Flavor Conservation in the charged lepton sector : processes like

m A → e A

m → e + g

m → e e e

have not been observed yet (down to 10^-13 !).

In SM (m_n = 0) Lepton Flavor is conserved absolutely (not by principle but by structure !) neutrino oscillations  m_n  0  Lepton Flavor is not anymore conserved (n oscillations)

 charged LFV possible via loop diagrams, but heavily suppressed m  e (or m   ) via n oscillations

measurement not affected by SM processes m  eee via a quantum loop

L  1 / M

_W

E

_n

 M

_W

 

4

54

4 10

W

m BR e e e M

n

m

^{  + - -}

    

 

sin (2 ) sin (^{2 2 2} ) 4

P m L

   E

n



n







2

New Physics in m → eee

several cLFV models predict sizeable effects,

accessible to the next generation of experiments !

if cLFV seen, unambiguous signal for new physics (going beyond Dirac m_n > 0)

explore physics up to the PeV scale

complementary to direct searches at LHC

Dipole (Loop Diagrams)

Supersymmetry Little Higgs Models Seesaw Models

GUT models (Leptoquarks) many other models …

Contact (Tree Diagrams)

Higgs Triplet Models

New Heavy Vector Bosons (Z’) Extra dimensions (K-K towers) many other models …

Model Comparison (m → eg and m → eee)

   

^,

2 2

1 1

dipole e ee

LFV

L m

^m

H  J J

_^{m }

 ⁺  

+ +

 

  0

  

effective charge LFV Lagrangian (“toy” model)

Kuno & Okada



= common effective scale



= “contact” vs “loop”

g* / Z*

g

explore physics up to PeV scale

de Gouvea & Vogel

 4

LFV m Decays : Experimental Signatures

kinematics :

2-body decay quasi 2-body decay 3-body decay monochromatic e⁺, g mono-energetic e^- coplanar, Sp_i = 0

back to back SE_i = m_m

backgrounds :

accidentals decay in orbit radiative decay antiprotons, pions accidentals

beam :

continuous beam pulsed beam continuous beam

none of these decays, however, have been yet observed experimentally

Mu3e @ PSI : the Challenge

search for m

⁺

 e

⁺

e

^-

e

⁺

with sensitivity BR ~ 10

^-16

(PeV scale)



_{(m  eee)}

> 1000 years (

_m

= 2.2 ms) using the most intense DC (surface) muon beam in the world (p ~ 28 MeV/c)

suppress backgrounds below 10

^-16

find or exclude m

⁺

 e

⁺

e

^-

e

⁺

at the 10

^-16

level

4 orders of magnitude over previous experiments (SINDRUM @ PSI)

aim for sensitivity / staged approach 10

^-15

in Phase I

10

^-16

in Phase II

(i.e. find one m

⁺

 e

⁺

e

^-

e

⁺

decay in 10

¹⁶

muon decays)

 observe ~10

¹⁷

m decays (over a reasonable time scale) rate ~2 x 10

⁹

m decays / s

 build a detector capable of measuring 2 x 10

⁹

m decays / s minimum material, maximum precision

project (Phase I) approved in January 2013

6

Mu3e Baseline Design – Phase I

thin (< 0.1% X

₀

), fast, high resolution detectors

(minimum material, maximum precision)

175 M HV-MAPS channels (Si pixels w/ embedded amplifiers) 10 k ToF channels (SciFi and Tiles)

acceptance ~ 70% for m

⁺

 e

⁺

e

^-

e

⁺

decay (3 tracks!)

solenoid B = 1 T

surface m

p ~ 28 MeV/c 10

⁸

m / s

~15cm

~1.2 m

Phase I

scintillating tiles scintillating fibers Si pixels (HV-MAPS)

Muons @ PSI

most intense DC muon beam

590 MeV/c proton cyclotron, 1.4 MW

pE5 beamline ~ 10⁸ m / s - surface muons ~ 28 MeV/c

- high intensity monochromatic beam (ΔP/P < 8% FWHM)

- polarization ~ 90%

(MEG exp., Mu3e phase I)

SINQ (spallation neutron source) could even provide 5 x 10¹⁰ m / s

High-intensity Muon Beamline (HiMB)

e / m 12 cm separation at last collimator

> 8σ separation

8

Mu3e – Phase I

MEGII and Mu3e (phase I)

have similar beam requirements and will share he same beam-line

Mu3e

MEG

pE5 beamline

can easily switch between the two experiments intensity O(10⁸ muon/s)

low momentum p = 28 MeV/c small straggling

good identification of the decay region Proof-of-Principle:

delivered 8 x 10⁷ muon/s during 2016 test beam

Signal and Backgrounds

n

e

n

m

signal backgrounds

internal conversion accidental

features

common vertex common vertex no common vertex Sp

_i

= 0, SE

_i

= m

_m

Sp

_i

 0, SE

_i

< m

_m

Sp

_i

 0, SE

_i

 m

_m

in time in time out of time



_vtx

< 300 mm rejecting the background requires 

_p

< 0.5 MeV/c



_t

< 0.5 ns

BR (m⁺  e⁺ e^- e⁺n_en_m) = 3.5 x 10^-5

10

Irreducible Background

m radiative decay with internal conversion

BR (m⁺  e⁺ e^- e⁺n_en_m) = 3.5 x 10^-5

high momentum and energy resolution required to suppress this background



_p

< 0.5 MeV/c and m

_m

< 0.5 MeV/c

²

m

⁺

 e

⁺

e

^-

e

⁺

n

_e

n

_m

fraction in signal region

as a function of m

_m

n

e

n

m

Sp

_i

 0, SE

_i

 m

_m

Background Suppression

background rejected with tracking and timing

(tracking alone not sufficient to reject accidental background)

12

The Pixel Tracker

central tracker: four layers re-curl tracker: two layers

minimum material budget: tracking in multiple scattering dominated regime

momentum resolution < 0.5 MeV/c over a large phase space geometrical acceptance ~70%

X/X

₀

per layer ~0.011%

re-curl stations central stations

Silicon Pixel Detector HV-MAPS

High Voltage Monolithic Active Pixel Sensors : HV-MAPS

readout logic and amplifiers embedded in the pixel n-well

thin active region (10 mm) → fast charge collection via drift

< 50 mm thickness

final pixel size 80 x 80 mm² final chip size 2 x 2 cm² time resolution < 15 ns efficiency > 99%

175 M pixels

radiation hard operated at –85 V

1 Gb/s LVDS readout (30 M hits /s)

Peric NIMA731 (2008) 131

14

HV-MAPS R & D

50 mm thick silicon wafer Latest prototype: MUPIX 8 ( MUPIX X)

characteristics thickness 50 mm

pixel size 80 x 80 mm² chip size 19 x 10 mm² performance

efficiency > 99 %

time resolution < 14 ns

full size

Timing 50 ns snapshot (readout frame): 100 m decays

additional ToF information < 500 ps

to suppress accidental backgrounds requires excellent timing

< 500 ps SciFis < 100 ps scint. tiles

16

The Timing Detectors: Fibers and Tiles

precise timing measurement: critical to reduce accidental BKGs

determine sign of re-curling tracks (SciFi) scintillating fibers (SciFi) ~250 ps, detection efficiency > 95 %

scintillating tiles ~70 ps, detection efficiency > 99 %

scintillating tiles scintillating fibers

The SciFi Detector

Requirements

high detection efficiency > 95%

time resolution < 0.5 ns

thickness X/X₀ ~ 0.2 % (< 700 μm)

handle high occupancy: up to 250 KHz/fiber limited space for electronics and cabling Design

cylindrical

~12 cm diameter length ~30 cm

3 staggered layers round fibers multi-clad 250 μm round fibers

readout with Si-PM arrays on both ends MuSTiC ASIC

18

SciFi Perforomance

different fibers have been evaluated

SCSF 78 MJ, SCSF 81 MJ, NOL 11, BCF 12 w/ and w/o TiO₂ coating, 3 & 4 layers, … detection efficiency > 96 % @ 0.5 phe thr

timing resolution ~200 ps (mean time)

time resolution light yield

full size SciFi ribbon prototype

Si-PM array 2 x 64 ch., 250 mm pitch common cathode

32.5 mm

Summary

Mu3e will search for the neutrinoless muon decay m → e

⁺

e

^–

e

⁺

with a sensitivity at the level of 10

^-16

i.e. at the PeV scale

 suppress backgrounds below 10

^-16

(16 orders of magnitude !) Novel technologies:

HV-MAPS (Si pixels, 50 mm thickness) Si-PMs (scintillating fibers and tails)

they meet the requirements

Staged approach

Stage I (2020 – 2024)

~10

⁸

m decays / s BR(m → eee) < 10

^-15

approved in January 2013

Stage II (> 2025)

~2 x 10

⁹

m decays / s BR(m → eee) < 10

^-16

HiMB feasibility study already started

Construction in 2018/2019 (incl. magnet) Commissioning 2020

20

LFV Searches : Current Situation

The best limits on LFV come from PSI

muon experiments m

⁺

→ e

⁺

e

^-

e

⁺

BR < 1 x 10

^-12

SINDRUM 1988 m

^-

+ Au → e

^-

+ Au BR < 7 x 10

^-13

SINDRUM II 2006 m

⁺

→ e

⁺

+ g

BR < 4.2 x 10

^-13

MEG 2016

Mu3e m

⁺

→ e

⁺

e

^-

e

⁺

Phase I : BR < 10

^-15

Phase II: BR < 10

^-16

SINDRUM

SINDRUM II MEG

SINDRUM @ PSI (~ 80s)

e⁺ spectrum m⁺ → e⁺2n 105.7 MeV

3e2 0

i i i i

K E

m

p c

m

m n







+





prompt events

beam (pE3 beamline @ PSI):

5 ´ 10⁶ m / sec

28 MeV/c surface muons resolution:

(p_T) = 0.7 MeV/c² vertex ~ 1 mm

statistics limited!

 

 ^{m m}

^{ee e e}

^{n n}

^{m e}



^{10 12 (90% CL)}

+ + - +

-

+ +

 

  

accidental events (normalized)

22